1. Field of the Invention
The invention relates to video terminals which are adapted inter alia to reproduce, starting from digital electrical video signals, incoming or outgoing information in the form of a two-dimensional image which is composed of characters or symbols.
2. State of the Art
Known designs of video terminals are generally provided with a reproducing unit comprising a screen, which unit reproduces, via the screen, a two-dimensional image consisting of a frame pattern of image lines; with a central processing unit with an associated program store; with an image store for the freely addressable reading in and out of image information; and with a control unit for controlling the access to the image store, for generating image store address combinations, each provided with an address part due to which the contents for an image line are determined, and for addressing the screen. Video signals which have the form of digital bit-patterns and which have to be processed by such video terminals, are supplied to the reproducing unit via a video output stage. The electromagnetic fields which are i.a. caused by the amplification of such digital video signals, are of such a strength that under certain circumstances it will be possible to receive these signals with a normal TV receiver and a suitable aerial, and to reconstruct the image from the received signal at a distance of abt. 800 m from the "radiating" terminal in question. Such a radiation is generated in video terminals as well as in normal TV receivers capable of receiving Videotex signals.
Now there is a need to exclude any person other than the user of a video terminal or Viditel receiver from making use of said radiation to take, in an illicit way, cognizance of the information reproduced on the screen of such a terminal or Viditel receiver.
There are known proposals to restrict the radiation of a video terminal by making use of an electromagnetic shield installed around the terminal, as well as of provisions made to filter all the cables that pass through the wall of the casing of the terminal. A solution of this kind is unattractive from a technical-economical point of view.
The object of the present invention is to provide relatively cheap provisions which are simple from a technical point of view and due to which a user of a video terminal will be protected against illicit "looking in" by other persons at the information reproduced on his screen.